The present invention relates to a cutting insert for chip forming machining tools, in particular milling cutters. The insert is preferably made by form pressing and sintering of an insert-forming powder. The insert comprises an upper side or chip surface, a lower, planar bottom surface which can be located into abutment with a cooperating bottom support surface of the machining tool, and at least three side surfaces extending between the upper and lower surfaces. At least one of the side surfaces adjoins the chip surface along a line that constitutes a cutting edge.
Such inserts are more and more produced by a direct pressing method during which a cemented carbide forming powder first is shaped into a desired form in a suitable pressing tool and then is given final strength by sintering in an oven at temperatures above 1000.degree. C. The pressing operation has been sophisticated over the years and is nowadays so well defined that the process provides good possibilities of forming the cutting edge and adjacent chip forming surfaces, possible reinforcing chamfers and clearance surfaces with great exactitude. Moreover, the shrinkage involved in the process is taken into account during the dimensioning of the pressing tool.
Today's cutting geometries tend to be more and more positive, i.e., larger and larger angles between the chip or rake surface of the insert and the normal to the machined surface. The reason for this geometry is that one obtains several advantages thereby, such as a small cutting force and thus a low energy consumption, a well defined cutting edge for high measure precision, and a wider liberty when selecting relief angle while maintaining a positive cutting geometry. In practice, the limit for the positivity of the chip surface is determined by the strength of the cemented carbide, since the edge angle becomes sharper, and thus weaker, the more positive the chip surface is.
Thus, a first object of the present invention is to make a cutting insert with a chip or rake angle as positive as possible in order to minimize the cutting forces.
A second object of the present invention is to make a cutting insert that reduces, or even eliminates, chipping and breakage of the cutting edge, while at the same time paying due attention to the first mentioned object.
The present invention has in a surprising way managed to satisfy both these, seemingly incompatible objects. This has been accomplished, in part, by forming the insert with a portion of the upper surface that extends along and adjacent to at least one cutting edge as a helically twisted rake surface, whereby the rake angle of this rake surface decreases from an operative cutting corner towards a point along the cutting edge. Thereafter, the rake angle increases from the point on the cutting edge towards a next adjacent cutting insert corner.
Thus, by forming the insert with the largest rake angle nearest to two adjacent cutting corners, and the smallest rake angle at a point somewhere along the cutting edge between these two corners, preferably at the middle of the cutting edge, several advantages have been attained. The total cutting force at large cutting depths has been reduced thanks to the increasing rake angle at maximal cutting depth. At the same time, chipping or breakage of the cutting edge is substantially eliminated even after long machining times. Further, the cutting edge is strongest at a point at which, or in whose proximity, the largest load will be exerted, i.e., at large cutting depths. In order to make the "average rake angle" as large as possible, and thereby the cutting force as small as possible, the rake angle is increased again towards the adjacent operative corner. Thus, by the present invention one has achieved a very positive cutting geometry with a high strength.